The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Reverse osmosis systems typically use one or more membrane housings that have one or more membranes therein that are used to extract an essentially pure fluid from a solution. The desalination reverse osmosis membranes receive feed fluid from brackish or sea water and extract fresh water therefrom. Fresh water is extracted or separated when the pressure of the feed fluid exceeds the osmotic pressure of the fluid which allows permeate or product fluid to cross the semi-permeable reverse osmosis membrane. The fluid that is left on the input side to the membrane becomes higher in salt concentration because fresh water that travels through the membrane does not include the salt. The water that passes through the membrane is referred to as a permeate. The pressure required to produce fresh water is proportional to the concentration of the total dissolved solids (TDS) in this feed solution within the reverse osmosis housing. For typical ocean water, the concentration is about 35,000 parts per million (ppm) and the corresponding osmotic pressure is about 450 pounds per square inch (psi) (3,102 kPa). For 70,000 ppm feed fluid, the osmotic pressure approximately doubles to 900 psi (about 6,205 kPa). A typical seawater reverse osmosis system uses a series of membranes that recover up to about 45% of the fresh water and generate about 55% concentrate brine from the original volume of seawater. The net driving pressure (NDP) equals the feed pressure minus the osmotic pressure. The net driving pressure is the pressure energy available to drive pure fluid across the membrane.
Referring now to FIG. 1A, a reverse osmosis system 10 according to the prior art includes a membrane array 12 that generates a permeate stream through permeate pipe 14 and a brine stream through a brine pipe 16 from a feed stream in a feed pipe 18. The feed stream originates from a source 19 typically includes brackish or sea water. A feed pump 20 coupled to a motor 22 pressurizes the feed stream 18 to a required pressure, and the feed stream 18 enters the membrane array 12 at the required pressure.
The membrane array 12 includes a membrane housing or pressure vessel 24 and a membrane 26. The portion of the feed stream 18 that flows through the membrane 26 before exiting the membrane array 12 forms the permeate stream 14 that exits through the permeate pipe 14. The portion of the feed stream 18 that does not flow through the membrane 26 before exiting the membrane array 12 forms the brine stream that exits in the brine pipe 16.
The permeate stream 14 is a purified fluid flow at a low pressure that collects in a tank 28 or is piped to a desired location. The brine stream 16 is a higher pressure stream that contains dissolved materials blocked by the membrane 26. The pressure of the brine stream 16 is only slightly lower than the feed stream 18. A control valve 30 may be used to regulate the flow through and pressure in the membrane array 12. The brine stream 16 may flow through the control valve 30 and into a drain or tank 32.
Referring now to FIG. 1B, the membrane 26 of FIG. 1A is typically formed of a plurality of elements 40. The elements 40 are typically formed in a cylindrical shape by rolling a plurality of sheets and spacers together. In this example a first sheet 42 and a second sheet 44 are glued together on three sides with the fourth side being in glued communication with the central collection tube 46 communicates permeate to a desired location as indicated by arrow 48. Brine which may also be referred to as reject 50 does not enter the collection tube 46. The sheets and the spacers glued between the membrane sheets 42 and 44 to allow the sheet 44 to stay slightly apart and allow permeate to flow to the collection tube 46. A second spacer sheet 54 is used to keep the membrane sheets slightly apart and allow the axial flow through the element and allow brine or reject 50 to flow therethrough.
Referring now to FIG. 1C, an inlet pipe 60 fluidically communicates fluid into the pressure vessel 24. A flow distributor 62 distributes fluid to the reverse osmosis elements 40A-40E rather than around the elements 40A-40E. The fluid distributor 62 spreads the fluid flow radially across the surface of element 40A. The seal 64 allows fluid from the distributor 62 to not circumvent the first element 40A. The flow continues through the elements 40A-40E sequentially. Permeate collection tubes 46A, 46B, 46C, 46D and 46E receive the permeate from each respective element 40A-40E. Connectors 66A-66D join successive collection tubes 46A-46E. An anti-telescoping device 68 may be used to maintain the position of the elements 40A-40E relative to the flow distributor 62. In most applications between three and eight elements are used. Five of which are used in this example. A brine exit pipe 70 is used to emit the brine from the pressure vessel 24. Permeate exit collection tube flows in a direction indicated by the arrow 48.
As the feed progresses from element to element, the amount of total dissolved solids increases until the brine exits the pipe 70. The osmotic pressure is mostly determined by the concentration of the total dissolved solids. Each succeeding element experiences a higher concentration and thus higher osmotic pressure and lower Net Driving Pressure than the preceding element. Consequently, each successful element has lower permeate production than the preceding element. A minimum Net Driving Pressure for sea water in an RO system is about 100 psi (689.5 kPa). An initial feed pressure must be substantially higher than the initial osmotic pressure to ensure sufficient Net Driving Pressure available toward the end of the array. A typical pressure may be about 800 psi (5516 kPa) while the osmotic pressure is about 450 psi (3103 kPa) which yields a Net Driving Pressure of 350 psi (2413 kPa) for the first element. At the end of the array the osmotic pressure may be 700 psi (4826 kPa) which reduces the Net Driving Pressure to 100 psi (689.5 kPa). A high initial Net Driving Pressure is wasteful because the pressure is much higher than needed for an optimal rate of permeate production. In an ideal situation, the feed pressure would steadily increase to compensate for the increasing osmotic pressure resulting in a constant net driving pressure throughout the array.
Another issue with reverse osmosis systems is polarization. Polarization is the formation of a stagnant boundary layer adjacent to the membrane surface where the concentration of salinity and foulant becomes very high. Polarization occurs when the flow velocity through the membrane elements is reduced to a certain value. Polarization typically becomes severe when flow velocity drops to below fifty percent relative to the inlet flow velocity of the first element. The typical amount of permeate that can be recovered is about fifty percent or lower and may have a typical range between thirty-eight and forty-five percent.
Referring now to FIG. 2A, one way in which to achieve higher permeate recovery is employing a first set of pressure vessels 210A, 210B which feed a second set of pressure vessels 210C. In this example, two pressure vessels are illustrated in a first stage 212 and a single pressure vessel is illustrated in a second stage 214. This type of configuration is referred to as a 2:1 array. Feed fluid enters a feed manifold 220 which is distributed between the pressure vessels 210A and 210B. The brine exits the pressure vessels 210A and 210B through a brine manifold 224 to pressure vessel 210C in the second stage 214. Permeate exits the pressure vessels 210A and 210B through a permeate manifold 228. The permeate manifold 228 is also in communication with the permeate generated in the pressure vessel 210C. The higher concentrated brine is removed from the pressure vessel 210C through a brine pipe 230. Of course, other types of array configurations are known such as a 3:2 and 4:3. For three-stage systems 6:4:2 configurations have been used. Two-stage systems have permeate recovery of about fifty percent to seventy-five percent. Three stage systems may also recover up to about eighty-five percent of permeate.
A second example of a two-stage system is illustrated in FIG. 2B. In this example, a boost pump 240 is used between the two stages. That is, the boost pump 240 is in communication with the brine manifold 224 and boosts the pressure in the brine manifold 224 to a desirable pressure to compensate the losses in the Net Driving Pressure that occur within the pressure vessels 210A and 210B of the first stage 212. Energy recovery devices such as turbochargers are known to be used in reverse osmosis systems to recover the hydraulic energy in a brine stream that exit the last stage and boosts the pressure of another stream such as the feed stream.
In FIG. 3A a pressure vessel 310 has a brine stream 312 that is directed to a turbocharger 314 that has a pump portion 316 and a turbine portion 318. The turbine portion 318 receives the highly pressurized brine stream 312 which in turn is used to drive the pump 316 that receives feed fluid from a high pressure pump 320. The high pressure pump 320 is driven by a motor 322. The turbine 318 may also be connected to a motor 330 through a common shaft 332. The motor 330 is driven by a variable frequency device 334. During operation, the feed fluid is pressurized to an intermediate level by the high pressure pump 320. The final pressure of the feed fluid is attained by the pump portion 316 of the turbocharger 314. The pump portion 316 provides the feed fluid to the pressure vessel 310 through a feed pipe 340. Permeate leaves the pressure vessel through the permeate pipe 342.
A variable frequency drive 324 is used to drive the pump 320. However, if a motor 330 and variable frequency drive 334 are provided at the turbine 318, the variable frequency drive 324 is not necessary. Tin some cases, motor 330 may act as a generator should the turbine portion 318 produce more power than needed by pump section 314 to generate the desired feed boost. Fluid that has been depressurized in the turbine portion 318 is received within a drain 349.
A reverse osmosis system having a first stage 350 and a second stage 352 is illustrated by FIG. 3B. The first stage 350 is illustrated having a first pressure vessel 353 while the second stage is illustrated having a second pressure vessel 354. The permeate from each stage 350, 352 is collected in a permeate manifold 356. A brine manifold 358 is in communication with a turbocharger 360 that has a pump portion 362 and a turbine portion 364. A turbine portion 364 receives the pressurized brine fluid from the brine manifold 358 and turns the pump portion 362 to pressurize the brine fluid within the brine manifold 366 exiting from the first pressure vessel 353. A motor 370 and variable frequency drive 372 may also be used in this configuration. The motor 370 is used to adjust the motor speed of the turbine portion 364 which in turn raises the speed and pressure output of the pump portion 362. Thus, the motor 370 may be used to increase or reduce the boost from the pump portion 362.
Referring now to FIG. 4A, a turbocharger 410 is illustrated having a pump portion 412 and a turbine portion 414. The turbine includes a main nozzle 416 and an auxiliary nozzle 418 which is controlled by an auxiliary valve 420. A bypass valve 422 is located outside of the turbocharger 410 and may be used to control the amount of fluid bypassing the turbine portion 414. The entire or nearly the entire amount of fluid directed toward the turbocharger 410 may be communicated to the bypass valve 422. A backpressure valve 424 is in fluid communication with the outlet of the turbine portion 414 and or the valve 422. The backpressure valve 424 is used to create a backpressure on the turbine portion 414. The flow through the pipe 430 is regulated by the bypass valve 422. The controller 426 controls the operation of the auxiliary valve 420 through control line 432A. The operation of the bypass valve through control line 432B and the control of the back pressure valve 424 through control line 432C. Typically, a system operator changes the settings to open and enclose the valves in the desired manner.
Referring now to FIG. 4B, the turbine differential versus turbine flow 450 has the auxiliary valve 420 in a closed position. Line 452 illustrates the differential pressure with the auxiliary valve open over a turbine flow range. Curve 454 shows the estimated feed pressure boost with the auxiliary nozzle in the fully opened position. Curve 456 shows the feed boost with the auxiliary nozzle fully closed. The graph illustrates 460, 462, 464 and 466. Area 460 is entirely within the auxiliary nozzle range and has the bypass valve 422 fully closed and the backpressure valve 424 fully opened. Area 462 extends into the region which the turbine cannot create enough flow resistance to achieve the desired ΔP at the indicated flow range. Therefore backpressure valve 424 must be partially closed. Area 464 extends into the bypass region that illustrates that the turbine cannot bypass the entire flow. Therefore, the bypass valve 422 and turbine is needed to handle the excess flow. In area 466 a portion is in the backpressure and a portion in the bypass region. Therefore, bypass valve 422 and the backpressure valve 424 are actuated appropriately. That is, the areas 462, 464, 466 are manipulated by the valves 422 and 424 so that the turbine operates between the curves 450 and 452.
Referring now to FIG. 5A, an energy recovery device 510 is illustrated having with a main inlet nozzle 512 that receives a brine stream. The inlet nozzle 512 communicates inlet fluid such as the brine stream to a volute 514. An auxiliary channel 520 is used to communicate fluid through an auxiliary nozzle 522. A valve stem 524 is manually operated to open and close the auxiliary nozzle 522. A seal 526 such as an O-ring or O-rings prevent leaking of brine toward the handle 528 and adjacent to the valve stem 524 from being communicated to the atmosphere. The valve stem 524 in FIG. 5A is shown in a closed position. According to FIG. 5B, the valve stem 524 is illustrated in an open position to show the valve seat 530 against which the end of the valve stem 524 seals. When additional turbine flow or a reduced differential pressure is required across the turbine the valve stem 524 may be withdrawn to create a flow path from the turbine inlet to the volute through the passage or auxiliary nozzle 522.